A study was conducted to evaluate the separate response to Lysine (Lys) and to Methionine (Met) in diets on live performance of young broiler chickens during the grower period of 14–35 d. Corn and soybean meal were used to formulate basal diets to provide 0.80 to 1.30% digestible Lys in increments of 0.10%. The mean of suggested amino acid ratios to Lys suggested by literature values was used in formulation according to the ideal protein concept. All amino acids other than Met and TSAA were calculated to meet or exceed the expected ratio to Lys. Experimental diets were prepared by addition of variable amounts of MHA (84% of Met) and cornstarch to the Lys basal diets to provide increments of 0.03% up to 0.21% supplemental Met activity for each level of digestible Lys. Two consecutive trials using the same experimental diets were conducted with identical design. Each of the 48 test diets was fed to 2 replicate pens of each trial at 14 d. Body weights by pen were obtained at 14, 28 and 35 d of age with feed consumption determined during the test period. During the period of 14 to 28 d, there were significant effects of the dietary Lys and added Met levels on BWG and FCR, with optimal digestible Lys level ≥0.90% for BWG and ≥ 1.30% for FCR. The optimal level of added Met during this period is ≥0.09% (total Met ≥ 0.33) and ≥ 0.12% (total Met ≥ 0.43) for BWG and FCR, respectively. During the period of 28 to 35 d, there were significant effects of the dietary Lys and added Met levels on BWG and FCR, with optimal digestible Lys level ≥1.00% for BWG and ≥ 1.30% for FCR. The optimal level of added Met during this period is ≥ 0.18% for both BWG (total Met ≥ 0.38) and FCR (total Met ≥ 0.49). No significant interactions between Lys and Met were observed based on BWG and FCR during each of the 2 periods. Results of this study suggest that the response to variation in Lys is a response to itself but not to Met in broiler grower diets.
